feat(text): MTEXT background fill / mask + text frame, editable in Properties

Render the MTEXT background (never drawn before — the flags were only shown
read-only in the panel) and make it editable.

- Text arm emits an opaque fill_tris rect behind the glyphs: flag 0x01 uses
  the entity background colour, 0x02 uses the canvas colour (a wipeout-style
  mask). Box = glyph bounds padded by (background_scale - 1) × text height.
- Text frame (flag 0x10) draws a closed rectangle in the text colour around
  the same box.
- Threaded `bg_color` into `tessellate()` (+ all callers and the
  MultiLeaderTess trait) so the mask can use the drawing-window colour.
- Properties: "Background mask" is a Off / Fill / Mask dropdown, "Background
  color" is its own colour picker (independent of the entity colour — new
  PropBgColor* messages; picking a colour switches to Fill mode), and "Text
  frame" is a checkbox toggle. Edits re-tessellate so the change shows live.

Renders via the face3d fill path, so the fill shows with FILLMODE on (same as
MLEADER background fill); the frame is a stroke and always shows.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/cache/block_cache.rs

@@ -429,7 +429,7 @@ fn tessellate_sub_local(
     // before casting to f32 — same precision-preservation trick used for
     // top-level entities, applied per-defn.
     let wires_out = tessellate::tessellate(
-        doc, h, sub, false, sub_color, pat_len, pat, lw_px, anno_scale, None,
+        doc, h, sub, false, sub_color, pat_len, pat, lw_px, anno_scale, None, bg_color,
     );
     if wires_out.is_empty() {
         return vec![];